Aqueous solution of zinc nitrite and method for production thereof

ABSTRACT

An aqueous zinc nitrite solution which contains substantially no calcium (Ca) ions is provided, in which, in terms of the aqueous zinc nitrite [Zn(NO 2 ) 2 ] solution having an NO 2  concentration of 10% by weight, the sodium (Na) ion concentration is 200 to 2000 ppm and the sulfate (SO 4 ) ion concentration is 20 ppm or less in the solution. The aqueous zinc nitrite solution can be prepared by providing a zinc compound and an alkali nitrite as raw materials and subjecting the raw materials to electrolytic synthesis through a double decomposition reaction using an ion-exchange membrane as a diaphragm. As the aqueous zinc nitrite solution is provided, an extremely efficient metal surface treatment is made possible which has a reduced amount of sodium ions. In particular, it contains substantially no sulfate ions and no calcium ions.

TECHNICAL FIELD

[0001] The present invention relates to an aqueous zinc nitrite solutionand a method for preparing the same. More particularly, the presentinvention relates to: an aqueous zinc nitrite solution making extremelyefficient metal surface treatment possible, where the amount of sodiumions is reduced and in particular a sulfate ions and calcium ions aresubstantially not present, making, in particular, a closed system offilm-forming treatment on metal possible; and a method for preparing thesame.

BACKGROUND ART

[0002] Zinc nitrite is known to decompose at around 100° C. with releaseof nitrogen oxide when slowly heated in air. Also, zinc nitrite is knownto be soluble in water and vulnerable to hydrolysis and forms zincoxynitrite [ZnO.Zn(NO₂)₂] upon evaporation of its aqueous solution.

[0003] Generally, such zinc nitrite is prepared by a method of mixingzinc sulfate with an ethanol solution of sodium nitrite, filtering theresulting precipitate, and evaporating and concentrating the filtrate toobtain crystals (see “Kagaku Daijiten” published on Mar. 15, 1984 byKYORITSU SHUPPAN CO., LTD.).

[0004] However, this method, which is a method in which sodium sulfateis separated and removed by utilizing the difference in solubilitybetween zinc nitrite and sodium sulfate, requires the use of ethanol,and the operations of evaporation and concentration. The method hasproblems in that it inevitably causes an increase in cost, sodium ionsare also allowed to remain, and the like.

[0005] Therefore, it is currently difficult to obtain high purity zincnitrite or its aqueous solution on an industrial scale.

[0006] Further, as a pretreatment process prior to painting of a metalsurface, a series of steps of degreasing, washing with water, filmforming treatment, washing with water, and drying is generally included.As an example of the film forming treatment, a treating method forforming a film of zinc phosphate on the surface of steel is commonlyadopted. As a film-forming agent used for this purpose, a treatingliquid prepared by dissolving zinc in phosphoric acid and diluting theresulting solution with water has been used. This treatment is termed“metal surface treatment”.

[0007] Furthermore, in order to promote the film forming reaction ofmetal, a chemical such as sodium nitrite or sodium chlorate is added tothe film-forming agent. These chemicals are called “promoters”. Additionof these promoters shows an effect that the forming treatment can beperformed at a lower temperature, and the film forming treatment time isreduced.

[0008] However, conventional sodium salts such as sodium nitrite andsodium chlorate have a problem in that long usage of a treating bathincreases the concentration of Na ions and as a result the pH of thetreating bath is elevated so that components of the formed filmprecipitate in the treating bath. Also, there is a problem in that, whenrecovering and regenerating the old treating liquid, accumulation of Naions in the treating bath destroys the balance of the bath so thatremoval of Na ions from the recovered treating liquid is necessary.Usually, treating liquid containing Na ions must be disposed of asindustrial waste.

[0009] Further, the issue of environmental protection has recentlyattracted much attention even in the field of metal surface treatingliquids, and attempts are being made to establish a closed system fortreating baths.

[0010] On this account, more intensive studies than ever before arebeing made of a metal surface-treating method with a lower generation ofsludge.

[0011] The present inventors proposed an aqueous zinc nitrite solutiondistinguished as a film-forming promoter for a metal surface treatment,which contains substantially no sodium ions and no sulfate ions, and isobtained by reacting between zinc sulfate and calcium nitrite at firstand then conducting purification (Japanese Patent Application No.2000-141893). In addition, it is known that the presence of calcium ionsin the film-forming promoter causes the promoter to become sludge ascalcium phosphate in the surface-treating solution, for example, at thetime of mixing with a zinc phosphate film-forming solution. Usually, thesludge is periodically recovered to be prevented from accumulating in atreating bath. However, the operation of collecting the sludge is alsocomplicated, thereby failing to show an industrial advantage.

[0012] The present inventors have been dedicated to making repetitivestudies of an aqueous zinc nitrite solution useful as a film-formingpromoter for a metal surface treatment, and as a result discovered anitrite aqueous solution having a reduced amount of sodium ions, inparticular, containing substantially no sulfate ions and no calciumions.

[0013] In other words, an object of the present is to provide an aqueouszinc nitrite solution that allows extremely efficient metal surfacetreatment and has a reduced amount of sodium ions, in particular,containing substantially no sulfate ions and no calcium ions, and amethod for preparing the same.

DISCLOSURE OF THE INVENTION

[0014] Under such circumstances, the present invention has beenaccomplished on the basis of the knowledge that the generation of sludgeis reduced and extremely efficient metal surface treatment is madepossible by using an aqueous zinc nitrite solution as a film-formingpromoter for the metal surface treatment, in which the aqueous zincnitrite solution containing substantially no sulfate ions and no calciumions while containing 500 to 2000 ppm of sodium ions is obtained byproviding a zinc compound and an alkali nitrite as raw materials andsubjecting the raw materials to electrolytic synthesis through a doubledecomposition reaction using an ion-exchange membrane as a diaphragm.

[0015] Thus, according to a first aspect of the present invention, thereis provided an aqueous zinc nitrite solution which containssubstantially no calcium (Ca) ions, characterized in that, in terms ofthe aqueous zinc nitrite [Zn(NO₂)₂] solution having an NO₂ concentrationof 10% by weight, sodium (Na) ion concentration is 200 to 2000 ppm andsulfate (SO₄) ion concentration is 20 ppm or less in the solution.

[0016] Also, according to a second aspect of the present invention,there is provided a method for preparing an aqueous zinc nitritesolution according to claim 1, characterized by including synthesizing azinc compound and an alkali nitrite as raw materials by a doubledecomposition reaction using an ion-change membrane as a diaphragm.

[0017] It is preferable that the reaction is performed in anelectrodialysis cell having a unit including one concentration chamberand two desalting chambers sandwiching the concentration chamber whichare formed by an alternate arrangement of cation-exchange membranes andanion-exchange membranes between an anode and a cathode.

[0018] Also, it is preferable that, in the method, an aqueous zinccompound solution is supplied to one of the desalting chambers and anaqueous alkali nitrite solution is supplied to the other of thedesalting chambers, and zinc ions are introduced through acation-exchange membrane and nitrite ions are introduced through ananion-exchange membrane into a concentration chamber sandwiched betweenthe desalting chambers, to thereby obtain the objective aqueous zincnitrite solution.

[0019] Also, according to a third aspect of the present invention thereis provided an aqueous zinc nitrite solution further including astabilizing agent.

BRIEF DESCRIPTION OF THE DRAWING

[0020]FIG. 1 is a schematic view showing the electrodialysis cell usedin the method for preparing an aqueous zinc nitrite solution of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] Hereinafter, the present invention will be described in detail.

[0022] The aqueous zinc nitrite solution of the present invention is asolution that includes a component represented by the general formulaZn(NO₂)₂ and H₂O mixed at any optional ratio.

[0023] In addition, even though industrial refining of the productobtained by the conventional manufacturing process is accompanied bymany difficulties, the aqueous zinc nitrite solution of the presentinvention is characterized in that the concentration of sodium (Na) ionsis reduced, in particular, substantially no sulfate ions (SO₄) and nocalcium (Ca) ions are contained.

[0024] Here, the concentration of zinc nitrite in the aqueous solutionis obtained by measuring the concentration of zinc (Zn) ions and theconcentration of nitrite ions, and provided as the concentration ofZn(NO₂)₂. The concentration of zinc ions is obtained by ICP luminescencespectrometry and the concentration of nitrite (NO₂) ions is obtained byion chromatography.

[0025] Furthermore, sodium (Na) ions, sulfate (SO₄) ions, and calcium(Ca) ions are all measured by ICP luminescence spectrometry. Here,sulfate (SO₄) ions are measured as sulfur (S) and converted to sulfateions.

[0026] Furthermore, the concentrations of sodium (Na) ions, sulfate(SO₄) ions, and calcium (Ca) ions in the present invention arecalculated by converting into 10% by weight in terms of NO₂. Theconcentration of sodium ions is 200 to 2,000 ppm, preferably 500 to 1500ppm. The concentration of sulfate ions is 20 ppm or less, preferably 10ppm or less. In the present invention, furthermore, the expression“contains substantially no calcium (Ca) ions” means that theconcentration of calcium (Ca) ions is 100 ppm or less, preferably 10 ppmor less.

[0027] In the aqueous zinc nitrite solution of the present invention,the concentration of nitrite ions is 5 to 15% by weight, preferably 9 to12% by weight, the concentration of Zn ions is 5 to 10% by weight,preferably 7 to 9% by weight, and the concentration of Zn(NO₂)₂ is 10 to25% by weight, preferably 15 to 20% by weight.

[0028] In the aqueous zinc nitrite solution of the present invention,sodium ions, in particular, sulfate (SO₄) ions and calcium (Ca) ions arereduced in concentration. Therefore, using this in the composition of afilm-forming promoter for a metal surface treatment allows the designingof an extremely efficient surface treatment system.

[0029] Next, the method for preparing the aqueous zinc nitrite solutionof the present invention will be described.

[0030] The method for preparing the aqueous zinc nitrite solution of thepresent invention is characterized in that a zinc compound and an alkalinitrite are, provided as raw materials and subjected to electrolyticsynthesis through a double decomposition reaction using an ion-exchangemembrane as a diaphragm in the aqueous solution.

[0031] The present invention is preferably carried out as follows. Thatis, in an electrodialysis cell provided with a unit composed of aconcentration chamber and two desalting chambers sandwiching theconcentration chamber formed by the alternate arrangement ofcation-exchange membranes and anion-exchange membranes between the anodeand the cathode. Each of the desalting chambers is composed of ananion-exchange membrane on an anode side and a cation-exchange membraneon a cathode side. An aqueous zinc compound solution as a raw materialis supplied to the desalting chamber on the anode side and an aqueousalkali nitrite solution is supplied to the desalting chamber on thecathode side. An electric current is applied to introduce zinc ionsthrough a cation-exchange membrane and nitrite ions through ananion-exchange membrane into a concentration chamber sandwiched betweentwo desalting chambers. Therefore, the objective aqueous zinc nitritesolution is obtained in the concentration chamber.

[0032] The aqueous zinc compound solution is an aqueous solutionprepared by dissolving a water-soluble zinc compound in water. The zinccompound includes, for example, zinc sulfate, zinc nitrate, and zincchloride, and such compounds may be used singly or in combinations oftwo or more. Among them, zinc sulfate is preferred from the standpointof commercial availability and low price.

[0033] The concentration of the aqueous zinc compound solution is notparticularly restricted but is preferably not higher than the saturationconcentration at room temperature, and specifically 0.5 to 2.0 mol/L,still more preferably 0.9 to 1.3 mol/L.

[0034] The aqueous alkali nitrite solution, another starting rawmaterial, is an aqueous solution prepared by dissolving a water-solublealkali nitrite in water. The alkali nitrite includes, for example,sodium nitrite, potassium nitrite, and lithium nitrite, and these may beused singly or in combinations of two or more. Among them, sodiumnitrite is preferred from the standpoint of commercial availability andlow price.

[0035] The concentration of the aqueous solution of a soluble alkalinitrite is not particularly restricted but is preferably not higher thanthe saturation concentration at room temperature, and specifically 1.5to 6.0 mol/L, still more preferably 3.0 to 4.5 mol/L.

[0036] The cation-exchange membrane that can be used in the presentinvention is not particularly restricted. For example, Selemion CMV(product of Asahi Glass Co.), Neocepta CM-1 (product of Tokuyama Co.),Nafion 324 (product of DuPont), and the like may be given. on the otherhand, the anion-exchange membrane is not particularly restricted and mayinclude Selemion AMV (product of Asahi Glass Co.), Neocepta AM-1(product of Tokuyama Co.), and the like.

[0037] The anode and cathode for use in the electrodialysis cell areeach made of a suitable material in a suitable shape depending on thematerial and an electrodialysis cell configuration, and morespecifically, include a metallic material such as platinum, iron,copper, or lead or a carbonaceous material.

[0038] The reaction temperature is 10 to 50° C., preferably 20 to 40° C.The current density is 1.0 A/dm³ to limiting current density, preferably1.5 to 5.0 A/dm³. The current time is 5 to 50 hours, preferably 10 to 40hours, but is not necessarily restricted to those conditions.

[0039] Further, detailed description will be made of the case where themethod for preparing an aqueous zinc nitrile solution is carried outusing the electrodialysis cell shown in FIG. 1.

[0040] In the electrodialysis cell shown in FIG. 1, from the anode sideto the cathode side, an anion-exchange membrane (A1), a cation-exchangemembrane (C1), an anion-exchange membrane (A2), and a cation-exchangemembrane (C2) are arranged in order. In addition, it is provided with ananode chamber/a desalting chamber (I)/a concentration chamber (II)/adesalting chamber (III)/a cathode chamber in that order.

[0041] The anode chamber and the cathode chamber are supplied with anelectrolyte such as Na₂SO₄, NaCl, or NH₄Br. Also, the desalting chamber(I) is supplied with the above aqueous zinc compound solution. On theother hand, the desalting chamber (III) is supplied with the aboveaqueous alkali nitrite solution. By applying the current, an aqueouszinc nitrite solution is produced in the concentration chamber (II).

[0042] The concentration of the aqueous zinc nitrite solution obtainedin the concentration chamber (II) rises as the current time is extended,but the sodium ion and sulfate ion concentrations of the solution interms of an aqueous zinc nitrite [Zn(NO₂)₂] solution having an NO₂concentration of 10% by weight also tend to rise. Therefore, it ispreferable to control the current time such that the sulfate temperatureion concentration will be 20 ppm or less and the sodium ionconcentration will be 2000 ppm or less.

[0043] In the aqueous zinc nitrite solution of the present inventionthus obtained, the concentration of nitrite ions is 5 to 15% by weight,preferably 9 to 12% by weight, the concentration of Zn ions is 5 to 10%by weight, preferably 7 to 9% by weight, and the concentration ofZn(NO₂)₂ is 10 to 25% by weight, preferably 15 to 20% by weight.

[0044] In addition, in terms of an aqueous zinc nitrite [Zn(NO₂)₂]solution having an NO₂ concentration of 10% by weight, the sodiumconcentration is 200 to 2000 ppm, preferably 500 to 1500 ppm, thesulfate ion concentration is 20 ppm or less, preferably 10 ppm or less,and the concentration of calcium is 100 ppm or less, preferably 10 orless. Therefore, the aqueous zinc nitrite solution of the presentinvention can be used as the composition of the film-forming promoterfor metal surface treatment.

[0045] Furthermore, according to the method for preparing the aqueouszinc nitrite solution of the present invention, an aqueous solution witha desired zinc nitrite can be obtained. In the present invention, whenthe sulfate ion concentration as an impurity in the solution in terms ofa zinc nitrite solution [Zn(NO₂)₂] having an NO₂ concentration of 10% byweight is larger than 20 ppm, the remaining sulfate ions may be removedand purified if desired.

[0046] The purifying process may include, for example:

[0047] (1) a method which includes adding barium ions to the solution toprecipitate the sulfate ions as barium sulfate;

[0048] (2) a method which includes passing the solution through acation-exchange resin or an anion-exchange resin; and

[0049] (3) a method which includes a solvent extraction procedure.

[0050] Method (1) is Preferred.

[0051] More specifically, it is sufficient to add a slight excess ofbarium ions relative to the residual sulfate ions and the additionamount relative to the residual sulfate ions may be, for example 1.05 to1.5 equivalents, preferably 1.05 to 1.2 equivalents.

[0052] The aqueous zinc nitrite solution of the present invention can bepreferably used as a corrosion inhibitor, in particular, a film-formingpromoter for metal surface treatment or compositions thereof. Thefilm-forming promoter is an ingredient that forms a film on the surfaceof a metal by being added into a film-forming solution.

[0053] The film formed at this time may be, for example, a zincphosphate film, an iron phosphate film, or a manganese phosphate film.When using the aqueous zinc nitrite solution of the present invention asa film-forming promoter, zinc phosphate film is particularly preferred.When the aqueous zinc nitrite solution is used as the zinc phosphatefilm, in the treating bath for forming the zinc phosphate film, thenitrite ions in the zinc nitrite have promoting effects similar to thatof the nitrite ions in sodium nitrite. In addition, zinc ions are amajor component of zinc phosphate film, so that the both anions andcations in zinc nitrite can exhibit their effect as surface treatingagents.

[0054] In addition, the aqueous zinc nitrite solution of the presentinvention is reduced in the concentration of sodium ions and issubstantially free of sulfate ions and calcium ions in particular. Thegeneration of sludge can be reduced when the aqueous zinc nitritesolution is used as it is as a film-forming promoter for the metalsurface treatment, or the aqueous zinc nitrite solution is used incombination with the conventional film-forming promoter. In addition,when a closed system of metal surface treatment is aimed at, anextremely efficient surface treatment on metal can be expected.

[0055] Other film-forming promoters for the metal surface treatment tobe used in combination are not particularly restricted but may include,for example, nitrous acid, sodium nitrite, nitrite ammonium,m-nitrobenzene sulfonic acid sodium, a hydrogen peroxide, sodiumchlorate, chlorate ammonium, nitric acid, soda nitrate, ammoniumnitrate, zinc nitrate, nitrate manganese, cobalt nitrate, calciumnitrate, nitrate magnesium, copper nitrate, and hydroxy amine. These areused alone or in combination with others.

[0056] When the aqueous zinc nitrite solution is used as a film-formingpromoter, in the range without affecting the characteristics of themetal surface treatment film and the characteristics of the metalsurface treatment film-forming agent, a stabilizing agent may be addedto stabilize the composition of the aqueous zinc nitrite solution byinhibiting the precipitation of crystals even at lower temperatures.

[0057] Such a stabilizing agent may include, for example, an alkalinitrite metal salt, sugar, or a chelating agent.

[0058] For the alkali nitrite metal salt, sodium nitrite, lithiumnitrite, or the like may be given.

[0059] For the sugar, sucrose, glucose, reduction malt sugar, mannitol,xylitol, starch, or the like may be given.

[0060] For the chelating agent, tartaric acid, gluconic acid, glycolicacid, glucuronic acid, ascorbic acid, citric acid, malic acid, ethylenediamine tetra-acetic acid, nitrilo triacetic acid, or glycine are given.

[0061] The above-mentioned stabilizing agents can be used alone or incombination with others. The addition amount of the stabilizing agent isusually 0.01 to 2% by weight with respect to the aqueous zinc nitritesolution.

EXAMPLES

[0062] The following examples illustrate the present invention infurther detail, but the present invention is not limited to theseexamples.

(Example 1)

[0063] Using a 5-cell electrodialyzer with ion-exchange membranes asillustrated in FIG. 1, only NO₂ ions and Zn ions were transferred froman anion-exchange membrane (Selemion AMV; product of Asahi Glass Co.)and a cation-exchange membrane (Selemion CMV, product of Asahi GlassCo.) to obtain an aqueous zinc nitrite solution. The experimentalprocess is as follows.

[0064] Dissolved in deionized water were 575 g of zinc sulfate 7H₂O toprepare a 15% by weight aqueous solution of ZnSO₄ and this solution wasfed to the desalting chamber (I). On the other hand, 600 g of sodiumnitrite were dissolved in deionized water to prepare a 30% by weightaqueous solution of NaNO₂ and this solution was fed to the desaltingchamber (III).

[0065] A 1.7% by weight aqueous solution of zinc nitrite was placed inthe concentrating chamber (II). The anode chamber and the cathodechamber were supplied with a 3.0% by weight aqueous solution of Na₂SO₄.Anion-exchange membranes (A1, A2) and cation-exchange membranes (C1, C2)each having an effective membrane area of about 120 cm² were alternatelyarranged as shown in FIG. 1. While the solutions were circulated withrespective pumps to maintain uniformity of the concentration of thesolution in each chamber, a voltage of 5 V was applied to theion-exchange membranes to carry out an ion-exchange double decompositionreaction for 40 hours, whereby an aqueous solution of zinc nitrite wasobtained. Note that, in the resulting aqueous solution of zinc nitrite[Zn(NO₂)₂], the concentration of zinc nitrite was 17.7% by weight and,assuming that the concentration of said aqueous zinc nitrite solution to10% by weight as NO₂, the sodium ion amount was 1188 ppm, the sulfateion amount was 10 ppm, and the calcium ion amount was 1 ppm.

[0066] The relationship between the concentration of Zn ions and theconcentration of NO₂ ions in the aqueous zinc nitrite solution of Theconcentration chamber (II) at dialysis time and also the relationshipamong the contents of sodium ions, sulfate ions, and calcium ions areshown in Table 1. TABLE 1 Concentration chamber (II) Concen- Concen-Concen- Concen- Concen- Concen- tration tration tration tration trationtration Mole of Current of Zn of NO₂ of Na of Ca of SO₄ Ratio Zn (NO₂)₂Time ions ions ions ions ions NO₂/Zn ions (hr) (ppm) (ppm) (ppm) (ppm)(ppm) (ppm) (wt %) 0 7444 9883 20 N.D. 0.7 1.89 1.7 1 10636 15191 117N.D. 2.4 2.03 2.5 10 34018 48000 448 N.D. 4.8 2.00 7.7 19 54875 76976693 N.D. 7.4 1.99 12.0 29 71118 102265 954 N.D. 7.6 2.04 15.4 40 84299119308 1188 N.D. 10 2.01 17.7

Examples 2 to 5

[0067] The concentration of zinc nitrite prepared by Example 1 was 17%by weight, and the stabilizing agents listed below were added, whileassuming that the concentration of the aqueous zinc nitrite [Zn(NO₂)₂]solution to be 10% by weight as NO₂, the sodium ion amount was 1188 ppm,the sulfate ion amount was 10 ppm, and the calcium ion amount was 1 ppm,followed by being sealed and left for 30 days at 5° C. in arefrigerator. After standing, the presence or absence of precipitate wasconfirmed by visual observation.

[0068] In the table, “absence” means that there was no precipitate.

[0069] Also, in Table 2, “Brending amount” indicates the addition amountwith respect to the aqueous zinc nitrite solution. TABLE 2 Stabilizingagent Blending amount Presence or absence of Kind (wt %) precipitateExample 2 Sodium nitrite 2 Absence Example 3 Starch 1 Absence Example 4Citric acid 0.5 Absence Example 5 EDTA 0.1 Absence

INDUSTRIAL APPLICABILITY

[0070] As described above, the aqueous zinc nitrite solution of thepresent invention is characterized by having a reduced concentration ofsodium ions, and particularly being substantially free of sulfate ionsand calcium ions in particular. The aqueous zinc nitrite solution isused as a film-forming promoter for metal surface treatment or thecomposition thereof. For instance, in the case of forming a zincphosphate film on the surface of steel or zinc by being added in asurface-treating agent of a zinc-phosphate system, the accumulation ofion impurities in a treating bath is small, the generation of sludge isalso decreased, and the frequency of replacing the liquid is greatlydecreased. Also, when a closed system is aimed at, an extremelyefficient metal surface treatment can be expected.

[0071] Also, according the method for preparing an aqueous zinc nitritesolution, the aqueous zinc nitrite solution can be prepared by themethod having an extreme industrial advantage.

What is claimed is:
 1. An aqueous zinc nitrite solution which containssubstantially no calcium (Ca) ions, characterized in that, in terms ofthe aqueous zinc nitrite [Zn(NO₂)₂] solution having an NO₂ concentrationof 10% by weight, sodium (Na) ion concentration is 200 to 2000 ppm andsulfate (SO₄) ion concentration is 20 ppm or less in the solution.
 2. Amethod for preparing an aqueous zinc nitrite solution according to claim1, characterized by synthesizing a zinc compound and an alkali nitriteas raw materials by a double decomposition reaction using an ion-changemembrane as a diaphragm.
 3. A method for preparing an aqueous zincnitrite solution according to claim 2, characterized in that thereaction is performed in an electrodialysis cell having a unit includingone concentration chamber and two desalting chambers sandwiching theconcentration chamber which are formed by an alternate arrangement ofcation-exchange membranes and anion-exchange membranes between an anodeand a cathode.
 4. A method for preparing an aqueous zinc nitritesolution according to claim 2 or 3, characterized in that an aqueouszinc compound solution is supplied to one of the desalting chambers andan aqueous alkali nitrite solution is supplied to the other of thedesalting chambers, and zinc ions are introduced through acation-exchange membrane and nitrite ions are introduced through ananion-exchange membrane into a concentration chamber sandwiched betweenthe desalting chambers, to thereby obtain an objective aqueous zincnitrite solution.
 5. A method for preparing an aqueous zinc nitritesolution according to any one of claims 2 to 4, characterized in thatthe zinc compound is zinc sulfate and the alkali nitrite is sodiumnitrite.
 6. An aqueous zinc nitrite solution according to claim 1,further comprising a stabilizing agent.
 7. An aqueous zinc nitritesolution according to claim 6, wherein the stabilizing agent is oneselected from alkali nitrite metal salts, sugars, and chelating agents.5. (Amended) A method for preparing an aqueous zinc nitrite solutionaccording to claim 2, characterized in that the zinc compound is zincsulfate and the alkali nitrite is sodium nitrite.